Characteristics of Mono-, Di-, and Trivalent Cations in Electric Double Layers: A Molecular Dynamic Investigation

Ionic behaviors, including ion distributions and hydration characteristics at solid-liquid interfaces, are important research interests in many important applications, such as electric double-layer capacitors and water lubrication. Here, we systematically investigated the concentration distributions, hydration numbers, and screening properties of Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, and La³⁺ ions inside electric double layers (EDLs) at various charge densities (σ). For σ weaker than -0.16 C/m², monovalent cations mainly accumulate in the outer Helmholtz plane (OHP). As the σ magnitude increases, monovalent cations start to dehydrate and migrate to the inner Helmholtz plane (IHP), following the order of K⁺, Na⁺, and Li⁺. This size-dependent behavior arises from the lower hydration energy of larger ions. While for the di- and trivalent ions, no obvious IHP appears. Based on ion distributions, the screening effect of counterions on surface charges is evaluated by analyzing the net charge distributions. As σ changes from 0 to -0.32 C/m², due to the stronger accumulation of cations in EDLs, the location of the neutral plane changes from ∼12 to ∼4 Å. When σ reaches a threshold, the excessive accumulation of cations can induce charge inversion. The threshold value and maximum reversed charge are found to correlate with the ion size, cation valence, and concentration.